Final answer:
The Aztec and Inca civilizations experienced drastic population declines because they had no immunity to European diseases such as smallpox. These diseases, brought by European explorers, led to severe epidemics that, along with societal disruption and malnutrition, devastated Native American populations. This situation allowed Europeans to establish dominance in the Americas.
Step-by-step explanation:
The Aztec and Inca civilizations suffered catastrophic population declines due to their lack of immunity to European diseases such as smallpox, influenza, and measles. These diseases were brought to the Americas by European explorers and conquistadors, leading to severe epidemics that devastated Native American populations who had no natural resistance. For example, diseases and the disruption of societies led to a dramatic reduction of the indigenous population on the Caribbean island of Hispaniola from over a million in 1492 to just 500 by 1548. In Mexico, smallpox introduced by the Spaniards had devastating effects on the Aztec capital Tenochtitlan, resulting in widespread death and the collapse of their society.
Malnutrition and the disruption of traditional lifestyles further weakened Native American immune systems, making them even more susceptible to diseases. In Tenochtitlan, after the outbreak of smallpox, the remaining population was too weakened to move or farm, leading to starvation. The Europeans' resistance to these diseases allowed them to take advantage of the weakened state of the indigenous peoples and to solidify their control over the Americas.